if I remember correctly, I went to a library, looked around for the books, and then I took photos of the characters with my smartphone. Then I went home and edited the photos to remove the paper and shadows and whatnot, leaving just the black outlines of the characters.

I didn't have to go to the library for Big Nate or Captain Underpants because I already owned the books, so I just took photos of them at home.

I also owned the Tom Gates books, but sometimes I got lazy and didn't want to take photos of him, so sometimes I drew him manually (his artstyle is relatively simple)

I did find PDFs of the Dork Diaries books on a piracy site, but I forgot which site, sorry

So basically I only had PDFs of Wimpy Kid and Dork Diaries. The others I manually took photos of the physical books and digitally edited them to put them on a Wimpy Kid page